The present invention relates to communication systems. More particularly, and not by way of limitation, the present invention is directed to a method and apparatus for allocating Rake fingers in a Direct-Sequence Code Division Multiple Access (DS-CDMA) receiver.
Direct-Sequence Code Division Multiple Access (DS-CDMA) is used in second and third generation digital cellular systems such as WCDMA, cdma2000, and IS-95. DS-CDMA is also used in certain Wireless Local Area Network (WLAN) systems. Base stations utilizing DS-CDMA experience an important resource allocation problem. The base station receives signals from many different users, and demodulation resources must be assigned to recover these signals. Traditionally, demodulation is performed by a Rake receiver, in which Rake fingers are allocated to different delayed images of the signal. These different images arise from the multipath propagation environment. Capturing as much signal energy as possible is important for several reasons. First, it lowers the required transmission power level of signals from users in the coverage area. This enables a user to transmit at a lower power, thus saving battery life. It also enables the base station to adequately receive signals from a user located at the outer edge of the coverage area when transmitting at maximum power. In addition, by lowering the required transmission power level, less interference is generated to other users. This allows more users to co-exist, increasing capacity.
While the description herein focuses on the base station and multiple users, a similar situation occurs at the mobile receiver and multiple links in the downlink. The mobile station must monitor various communication links from one or more base stations, allocating fingers to paths corresponding to these different signals. Thus, the terms “user” and “link” are used interchangeably in the description herein.
The standard approach to Rake finger allocation is to give each user a fixed number of Rake fingers. For example, the CSM2000™ base station chip provided by Qualcomm gives each uplink user four Rake fingers. However, allocating each user a fixed number of Rake fingers is not always efficient. One user may not use all the Rake fingers available to it, while another user could benefit from additional Rake fingers. Also, it may be more important to allocate more Rake fingers to certain users to keep the overall received power low.
It would be advantageous to have a method and apparatus for allocating Rake fingers that solves the shortcomings of existing methodologies. The present invention provides such a method and apparatus.